Restriction in an exhaust is necessary to an extent, unless your vehicle is a race-only rig or turbocharged. If you have no restriction, you will not get the scavenging effect. Think of dropping a marble in a glass of water. The marble "pushes" the water it hits downward, but the water around is "pulled" in after the marble because of the vacuum created by the pressure of the marble falling in. The pressure of the exiting combustion cycle will create vacuum behind it that will pull in your next intake charge. If you have an exhaust with no restriction, the exiting combustion pressure will dissipate before it creates enough vacuum to pull in your intake charge. This will cause a low rpm rough idle and or stall. You will have to increase idle speed to avoid this, or put a more restrictive exhaust to create more of a vacuum effect. The less restrictive exhaust will create a scavenging effect at high RPMS, but low RPM performance will suffer. The opposite is true for a more restrictive exhaust. However, too much restriction will cause everything to suffer.
I respectfully disagree.
